PowerShell: remotely verify if TCP port is listening

I recall one of the most powerful lessons I learnt on the job was when I learnt how to remotely (or locally – verify if a TCP port was actively listening using the TELNET command. So many ‘Is the firewall open?’ questions answered with a single command

More recently I’ve been in situations where I have a company assigned Windows notebook, with telnet.exe missing and various GPO restrictions in place. However, these notebooks did have PowerShell – so I became curious to determine if I could achieve the same verification as I was with Telnet – Yes you can. Open PowerShell – and use this:
If (New-Object System.Net.Sockets.TCPClient -ArgumentList '<fqdn.domain.tld>',<port-num>) { Write-Host 'YES' } If ($? -eq $false) { Write-Host 'NO' }

Replace <fqdn.domain.tld> with the Fully Qualified Domain name or IP address of the host to target.

Replace <port-num> with the target TCP port to probe if listening.

The script will respond with ‘YES’ if it’s listening, and ‘NO’ if it’s not.

Windows 10 – Reveal saved WiFi password as unprivileged user

Home/Sufficiently privileged users can typically get the WiFi password of an Access Point that they are currently connected to by doing the following:

  1. Connect to the WiFi AP you want to un-mask the password for
  2. Right-click the WiFi symbol in the systray and select ‘Open Network and Sharing Center’
  3. Click ‘Change adapter settings’
  4. Right click the WiFi adapter
  5. In the WiFi Status dialog, click ‘Wireless Properties’
  6. Click the Security tab and then check ‘Show Characters’ – this should show your current Wifi password

However, there are cases where this option isn’t viable – either the machine is locked down and doesn’t let you select ‘Show Characters’ – or you are in a situation where you are not in range of the WiFi AP to actually connect. Try this via the Windows command prompt (cmd.exe):

Run this command to list the wifi profiles the machine knows about:

netsh wlan show profiles

Then show the password for a specific profile:

netsh wlan show profiles name=<profile> key=clear

Replace <profile> with your profile name / SSID.

The password should be shown in Key Content: field

Home Brew – CO2 Carbonation Chart

Use this chart to determine the ‘set and forget’ settings (temperature and pressure) to apply to your kegged beer to properly carbonate it with a CO2 pressure tank. There are a couple of these around on the net, but usually limited to degrees fahrenheit and a few other annoyances for me, so I took a couple of minutes to put together mine own slightly more simplified version.

Carbonation Chart

Virtualize Raspberry Pi (i.e. ARM) on OS X using QEMU

Here’s how:

  • Install and upgrade Xcode to 4.3 or above.
  • Install the Xcode Command Line Tools (you can do this from within Xcode’s “Downloads” preference pane).
  • Install Homebrew, using the instructions here – http://brew.sh/
  • Force Homebrew to install version 1.1.0 of QEMU
    git checkout 2b7b4b3 Library/Formula/qemu.rb


Selective VPN routing [Solution: DSVR]

Before sharing about what I believe is my best solution yet, i’ll take a walk down memory lane…

Client VPN

Got my first VPN account (PPTP/L2TP) and happily used it from my various Windows/Mac/iOS clients. Very quickly I came across a few limitations, namely:

1) Only one device at a time could use the VPN.
2) More restricted clients did not have a client side capability to configure the VPN (like the Apple TV/PS3)
3) When a device was on the VPN, all traffic went down it – not ideal from a performance perspective, and when you’re location is determine by IP (e.g. Google Maps).
4) Minor at the time, but could not have multiple VPNs running.

Router VPN [+Source Based Routing]

Configuring  VPN connection at a single aggregation point (i.e. the router) was the next step, as this did solve some of the limitations (1 & 2 above). After a bunch of research (read: trial/error) I concluded that there was no practical way to be selective at the router based on destination as many services used over the VPN where using CDNs with massive network ranges which we’re not practically predictable.

I decided that source based routing was a good solution, in my case I selected my Apple TV to be the only source to push down the VPN this has served me well,  and I published a tutorial on it a while back.

Defining the next step

Even though my problem/use-case had largely been solved, I wanted to go further. I set myself the following goals for my next solution:

1) Selective destination routing, route only sites/domains I want.
2) Multiple concurrent VPN client support, each with their own destination mappings.
3) Multiple clients able to share the outgoing VPN(s) connection(s)
4) A ‘plug-n-play’ appliance, with no configuration changes to exiting infrastructure (clients/router/modem)
5) Simple web administration interface.

With those goals in mind – it was time to determine a solution.

New Solution: DSVR [Domain-Specific VPN Router]


1. Selective destination routing

I’ve already touched on some of the challenges (e.g CDNs) – there were a few more – but ultimately I determined that a pre-populated list of IP routes for a service will not work.

Then an idea came to me – what if I made something that I’d best describe as a ‘DNS Router’, at a high-level it would need to do the following:

1) Intercept all outgoing DNS queries from my clients.
2) Analysis the query name against which domains I’m interested in pushing down the VPN.
3) Pass the unmodified DNS request upstream to a valid DNS server, and get a valid response back.
4) Before passing the DNS response to the original client, add routes for the RR data within the DNS record

What does this do? For each matched DNS request we are creating static routes down the VPN!

Theory is great – but I needed to now make this thing.

It was clear to me that the best starting point was to leverage existing work that’s been done creating a DNS proxy. I found a great one – dnschef – by Peter Kacherginsky. Using this as a strong foundation I was able to modify and expand this to perform the 4 points above, and also handle some edge cases around CNAME responses etc.

In my testing – this has proven to work and be stable.

2. ‘Plug-n-play’ appliance, no configuration changes, clients able to share

Deciding on a hardware platform for the appliance was easy – Raspberry Pi – it’s low-cost, low-power, runs linux and small. It’s not without it’s limitations though (e.g. NIC limited by USB bus)

The aggregation point I selected to place the Raspberry Pi was between the existing CE/Modem and home router. This meant the RPi had to be configured as a typical NAT router. Other than the OS level configuration required to create the router, the RPi only has a single NIC, so I added a second NIC interface using a USB add-on.

To achieve the second portion of this goal ‘no configuration changes to exiting infrastructure’, I used iptables to transparent redirect all outbound DNS queries to my new custom DNS proxy/router.


3. Concurrent VPN client support

This was wasn’t too tough (for PPTP), basically just needed to configure the following for each:

1) PPP peer file
2) IP-UP script
3) INIT.D script

OpenVPN support is in the pipeline.

4. Web administration

Seeing that the core capability was done in Python, it was a logical choice to create the web interface in the same. I selected the Flask web-framework, and I used there small dev web server in lieu of the big boys like Apache.

The web interface allows for the following functions:

1) PPTP VPNs – Add/Modify/Delete
2) Specify which wildcard domains should route down which VPN
3) Stop/Start/Restart the core DSVR service
4) Status display for PPTP tunnel status, IP assignments, Route counts, uptime, mem/cpu usage
5) Reboot the RPi

webadmin (1)

Get/Build it

This blog post is just a static introduction. The code, instructions, limitations, issues and TODO’s are being maintained on the DSVR github page.

Do note that this should be considered an ALPHA release – it has not been tested by many people – use at your own risk.


Home Brew – Freezer to fermentation chamber conversion

For my home brew endeavors I had another problem to solve… a temperature controlled chamber for fermentation.

I had the following constraints/requirements:

1. Get it cold enough (10 – 15℃) for lager yeast. I live in a hot climate.
2. Large enough to allow my 30 litre fermenter (H:530mm,Dia:300mm) or my 6.5gal glass carboy (H:520mm, Dia:305mm) to fit.
3. Rather precise  temperature accuracy (~0.5℃)
4. Not cost “a lot”.

I decided that buying a suitably sized chest freezer and putting in a digital thermostat would be the way to go – Here’s what I bought:

1. Chest Freezer – Farfalla FCF-128A
– $299
– 128 litres
– External dimensions (W:750, H:780, D490)
– Available area for full-size fermenter (H:580, W: 382) (i.e. not including space above the “step”/”bump” inside the freezer due to the compressor)

2. Digital Thermostat – FOX1004

– $80
– Range 40.0 to 90.0℃
– 1 relay, 250 VAC 2A. Note: Not rated high enough to take full compressor load, hence the need for the contactor
– External dimensions (W:770, H:350, D:770)
– Input sensor diode(wire length : 3 metres)
– Input 230VAC 50/60Hz

3. Contactor – Schneider LC1K0901M

– $25
– TeSys K contactor
– 3P(3 NO) – AC-3 – <= 440 V 9 A
– 220…230 V AC coil

Now time to wire everything up.

Here is how I did it (click on the image to make it larger)

CircuitDiagram v0.2

The finished product


Note the thermometer probe is in the bowl filled with liquid – this will ensure a closer match to the actual beer temperature. Also, I put a battery powered (for now) fan in the chamber for circulation – need put in a better solution.